Microrelief of Rounded Diamond Crystals as an Indicator of the Redox Conditions of Their Resorption in a Kimberlite Melt

Alexander F Khokhryakov,Alexander G Sokol,Denis V Nechaev

doi:10.3390/cryst10030233

Alexander F Khokhryakov, Alexander G Sokol + Show 1 more

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https://doi.org/10.3390/cryst10030233

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Abstract

We conducted a detailed study of the morphology of diamond crystals partially dissolved in a water-bearing kimberlite melt at pressure of 6.3 GPa, temperature of 1400 °C, and two oxygen fugacities (fO2) corresponding to the Re-ReO2 buffer and near the magnetite–hematite (MH) buffer. The triangular etch pits on the {111} faces, which formed during experimental diamond dissolution, were found to completely correspond to negative trigons on natural diamond crystals in the shape and sidewalls inclination angle. Furthermore, two experimental fO2 values were associated with two relief types of the rounded tetrahexahedroid surfaces typical of natural rounded diamonds. Therefore, the surface microrelief on rounded natural diamond crystals was concluded to be an indicator of the redox conditions of natural diamond resorption.

Highlights

At present, no one doubts that the round shape and most surface patterns of natural diamond crystals were formed during the natural resorption process
Even a quick look at the crystals reveals significant differences in the morphology of diamond dissolution forms produced in a water-bearing kimberlite melt under conditions controlled by Re-ReO2 buffer (Figure 1a,b) and under more oxidizing conditions (Figure 1c,d)
Upon diamond dissolution in a water-bearing kimberlite melt at fO2 of the Re-ReO2 buffer, the octahedron edges are truncated by surfaces with well-defined striation relatively uniformly spread through the entire tetrahexahedroid surface

Summary

Introduction

No one doubts that the round shape and most surface patterns of natural diamond crystals were formed during the natural resorption process. Most researchers believe that the characteristic features of the composition of primary kimberlite magma are high MgO concentrations, but relatively small amounts of SiO2 (25–35 wt%), Al2O3, and alkalis [6,7,8]. Kamenetsky et al [9,10] believes that primary magma was almost anhydrous rich alkalis and halogens carbonate melt. The Udachnaya-East kimberlite can act as an adequate model of primary kimberlite melt. It should be noted that kimberlite melts can be a medium for the resorption of diamond, and for its crystallization [11,12]

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